Microwave applicator for endometrial ablation

ABSTRACT

A microwave applicator for applying electromagnetic radiation at microwave frequency includes a coaxial input for a microwave signal input and a waveguide for receiving and propagating the microwave signal input. Dielectric material is positioned within the waveguide and extends beyond the waveguide to form an antenna for radiating microwave energy. The coaxial input has direct in-line transition to the dielectric-filled waveguide. Preferably, this direct in-line transition is achieved by the central conductor of the coaxial input extending axially centrally into the waveguide so as to excite microwaves in the waveguide. A lateral conductor extends radially from the central conductor to assist the launch of the microwaves into the waveguide. Preferably, the applicator includes a temperature sensor which is directly connected to the coaxial input.

TECHNICAL FIELD

This invention relates to a microwave applicator for the treatment of abody by means of microwave electromagnetic energy. The body ispreferably biological tissue and, preferably, the applicator is for usein the treatment of menorrhagia.

Menorrhagia is a common condition in women over the age of forty andmanifests itself as excessive bleeding from the endometrium whichconstitutes the inner wall of the uterus.

The most common form of treatment is to carry out a hysterectomy inwhich the entire uterus is removed.

In our earlier application published under number WO95/04385, thecontents of which are incorporated herein by reference, we disclosed aprobe for applying electromagnetic radiation at microwave frequencywhich comprised a dielectric-filled waveguide with an exposed portion atthe tip defining an antenna. However, in several of the embodiments, themicrowaves were launched in a first air-filled waveguide and then themicrowaves were passed into a second waveguide which contained thedielectric material. Between the waveguides, a tapered waveguideprovided a transition. The dielectric filled waveguide was of smallerdiameter than the air-filled waveguide because, at a given frequency,the wavelength in dielectric is shorter. Hence the diameter of theapplicator in wavelengths remains constant throughout transition.

However, although such a applicator is perfectly satisfactory, theapplicator bandwidth is comprised by the resonance found in the longlength of dielectric filled waveguide. This means that any change infrequency generated by the microwave source could make a significantdifference in applicator efficiency.

SUMMARY OF THE INVENTION

According to the present invention, there is provided a microwaveapplicator for applying electromagnetic radiation at microwave frequencycomprising a coaxial input for receiving and passing a microwave signalinput of predetermined frequency, a waveguide for receiving andpropagating the microwave signal input, dielectric material positionedwithin the waveguide and extending beyond the waveguide to form anantenna for radiating microwave energy, characterised in that thecoaxial input has means for providing a direct, in-line transition ofthe microwave signal input into the dielectric-filled waveguide.

Preferably, this direct in-line transition is achieved by the centralconductor of the coaxial input extending axially centrally into thewaveguide so as to excite microwaves in the waveguide. A lateralconductor extends radially from the central conductor towards the outerwall of the waveguide and serves to assist the launch of the microwavesinto the waveguide in the appropriate mode for transmission to the tip.

Preferably, the applicator includes a temperature sensor which isdirectly connected to the coaxial input to minimise wiring.

Suitable, where the applicator is to be used for medical treatment suchas endometrial ablation, it is important that the applicator be sterilefor each use. Accordingly, preferably the applicator is coated with amicrowave transparent coating allowing the applicator to be cleaned inconventional manner.

Although the microwave applicator of the present invention may be usedfor any desired application, it is preferred that it be used forendometrial ablation. This requires applying microwave energy to theapplicator at a frequency which will be substantially completelyabsorbed by the endometrium, monitoring the operating temperature toensure that the endometrium tissue is coagulated evenly through theuterine cavity, thus maintaining the application of the microwave energyfor a period of time sufficient to destroy the cells of the endometrium.

The use of microwave power to heat the endometrium has two mainadvantages. Firstly, electromagnetic radiation at microwave frequenciesis strongly absorbed by tissue and at around 8-12 GHz all microwavepower is absorbed in a layer of tissue about 5 mm thick and it isimpossible for microwave heating to extend beyond this region. This isideal for the treatment of the endometrium which is about 5 mm thick.Secondly, because of this strong absorption, the amount of powerrequired to achieve the desired temperature is relatively small.

Moreover, the improved applicator of the present invention has thefollowing major advantages over the applicator previously disclosed inour aforementioned earlier application:

(i) the waveguide is shorter because, by forming a hybrid between acoaxial input and a dielectric filled waveguide, the distance betweenthe transition and the radiating tip is very much shorter. This, inturn, reduces the amount of dielectric material necessary which improvedband width and applicator efficiency; and

(ii) it is possible to make the applicator flexible.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example with reference tothe accompanying drawings, in which:

FIG. 1 is a diagrammatic side elevation of a preferred microwaveapplicator in accordance with the invention; and

FIG. 2 is a diagrammatic plan view of the waveguide of FIG. 1 showingthe microwave fields.

FIG. 3 shows the applicator used to perform endometrial ablation.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, a microwave applicator (1) has a circular section waveguide(2) filled with a dielectric material (3). The waveguide (2) terminatesshort of the end of the applicator (1) and a portion (4) of thedielectric extends therefrom to form a radiating antenna tip for themicrowave energy. That end of the waveguide remote from the tip (4), isconnected to a coaxial cable (5) that powers the waveguide. The innerconductor (6) of the cable (5) extends axially into the dielectric (3)along the axis of the waveguide (2) so as to directly excite microwavesin the waveguide (2). The outer conductor (17) of the cable (5) isconnected to the outer conductor wall (7) of the waveguide. Theconductor (6) terminates within the waveguide, and a lateral conductor(8) extends radially from the conductor (6) through the outer wall (7)and serves to cause the microwaves to launch into the dielectricmaterial (3) with the magnetic fields (14) and electric fields (15)oriented as shown in FIG. 2.

The coaxial cable (5) may be air-filled, but as illustrated in FIG. 1,it is filled with a dielectric (16), but this terminates short of thedielectric (3) of the waveguide (2) so as to leave an air gap (18) thataccommodates axial expansion of the dielectric (16) when the applicatoris heated in use, either during treatment or sterilisation.

The axial dimension L₁ of the air-gap (18), and the axial dimensions L₂and L₃ of the conductor 6 within the waveguide (2) either side of theconductor (8), are all selected to tune out the reactance of the loopformed by the conductor (8), and thereby reduce backward reflections andenhance forwards launching of the microwaves in the waveguide.

The conductor (8) is insulated by insulation (9) as it passes throughthe outer waveguide wall (7).

Also shown in FIG. 1 is a thermocouple (10) on the outside of theradiating tip (4) for sensing the operating temperature. Moreover, inorder to avoid additional wiring, the thermocouple (10) is directlyconnected by a connection 19 to the outer conductor (17) of the coaxialcable (5) at (11) and by a connection (20) outside the wall (7) to thecentral conductor (6) of the cable (5) via the lateral conductor (8) anda connection (12) at its outer end. Accordingly, the thermocouple signalpasses out on the same coaxial cable (5) bringing the microwave power tothe radiating tip (4). Conventional circuitry (not shown) is used tosense and extract the DC signal from the coaxial cable.

Although not shown, the applicator (1) is provided with amicrowave-transparent protective coating of PTFE or other suitablematerial. The temperature sensing thermocouple (10) is provided betweenthe coating and the dielectric material as well as being insulated fromthe dielectric material.

The preferred use of the applicator of the present invention asdisclosed in our aforementioned published application number WO95/04385where the applicator is supplied with a microwave frequency input in themicrowave spectrum, preferably in the region of 8-12 GHz, from amicrowave frequency generator source and amplifier.

FIG. 3 of the drawings shows the applicator 1 in use to performendometrial ablation. The applicator is shown inserted into a uterus 31with the tip 4 of the applicator in contact with the endometrium 32within the uterus and transmitting microwave energy to produce localheating over a substantially spherical region 33 of the endometrium soas to destroy cells of the endometrium in this region.

What is claimed is:
 1. A microwave applicator for applyingelectromagnetic radiation at microwave frequency, the applicatorcomprising: a waveguide with an outer waveguide wall enclosingdielectric material which extends beyond an output end of the waveguidewall to radiate microwave energy; and a coaxial input comprising aninner conductor and outer conductive sleeve surrounding said innerconductor for inputting a microwave signal of predetermined frequency atan input end of the waveguide, wherein the inner conductor extends fromthe outer conductive sleeve longitudinally within the waveguide wallinto the dielectric material and terminates at a free end thereof withinthe dielectric material, and a lateral conductor is connected to, andextends laterally from, the inner conductor at a point within thedielectric material spaced a predetermined distance away from said freeend so that the current flow in said inner conductor and lateralconductor launch microwaves in a fundamental mode within the dielectricmaterial that travel to the output end of the waveguide.
 2. A microwaveapplicator as claimed in claim 1, in which the inner conductor extendsalong the central axis within the waveguide.
 3. A microwave applicatoras claimed in claim 1, in which the lateral conductor extends as far asthe waveguide wall.
 4. A microwave applicator as claimed in claim 1, inwhich the lateral conductor is located in a central region along thelength of the inner conductor within the waveguide.
 5. A microwaveapplicator as claimed in claim 4 in which the lateral conductor extendsthrough an aperture in the waveguide wall and is electrically insulatedfrom the waveguide wall.
 6. A microwave applicator as claimed in claim1, in which the coaxial input is a dielectric filled cable in which thedielectric of the dielectric filled cable terminates short of thewaveguide to leave an air-gap.
 7. A microwave applicator as claimed inclaim 1, in which a sensor is mounted on the applicator, and the sensorsignal output is connected to the coaxial input.
 8. A microwaveapplicator as claimed in claim 1, which is adapted for medical use.
 9. Amicrowave applicator as claimed in claim 8 which is adapted for use asan ablator.
 10. A microwave applicator as claimed in claim 1 in whichthe waveguide is a circular section waveguide.
 11. A microwaveapplicator as claimed in claim 1 in which the lateral conductor isconnected to the inner conductor at a position so as to enhance transferof microwave energy to the waveguide.
 12. A medical microwave applicatorfor applying electromagnetic radiation to a target mass of biologicaltissue at microwave frequency, the applicator comprising: a waveguidewith an outer waveguide wall enclosing dielectric material that extendsbeyond an output end of the waveguide wall and is configured to radiatemicrowave energy; and a coaxial input comprising an inner conductor andouter conductive sleeve surrounding said inner conductor configured toinput a microwave signal at an input end of the waveguide that is of afrequency that will cause emitted microwave energy to be absorbed by thetarget mass of tissue, wherein the inner conductor extends from theouter conductive sleeve longitudinally within the waveguide wall intothe dielectric material and terminates at a free end thereof within thedielectric material, and a lateral conductor is connected to, andextends laterally from, the inner conductor at a point within thedielectric material spaced a predetermined distance away from said freeend so that the current flow in said inner conductor and lateralconductor launch microwaves in a fundamental mode within the dielectricmaterial that travel to the output end of the waveguide.
 13. A medicalmicrowave applicator as claimed in claim 12, in which the coaxial inputis configured to input a microwave signal at the input end of thewaveguide that is of a frequency that will cause emitted microwaveenergy to ablate the target mass of tissue.
 14. A method for ablatingbiological tissue in a body, the method including the steps of:providing a microwave applicator comprising a waveguide with an outerwaveguide wall enclosing dielectric material that extends beyond anoutput end of the waveguide wall and is configured to radiate microwaveenergy, and a coaxial input comprising an inner conductor and outerconductive sleeve surrounding the inner conductor for inputting amicrowave signal of predetermined frequency at an input end of thewaveguide, wherein the inner conductor extends from the outer conductivesleeve longitudinally within the waveguide wall into the dielectricmaterial and terminates at a free end thereof within the dielectricmaterial, and a lateral conductor is connected to, and extends laterallyfrom, the inner conductor at a point within the dielectric materialspaced a predetermined distance away from the free end so that thecurrent flow in the inner conductor and lateral conductor launchmicrowaves in a fundamental mode within the dielectric material to theoutput end of the waveguide; positioning the applicator in sufficientlyclose proximity to a target biological tissue mass to cause the tissuemass to absorb microwave energy emitted from the applicator; and causingthe applicator to emit electromagnetic radiation at a predeterminedmicrowave frequency that will cause the target tissue mass to absorb themicrowave energy.
 15. The method of claim 14 in which the step ofpositioning the applicator includes inserting the applicator into a bodycavity leading to the target tissue mass.
 16. The method of claim 15 inwhich: the step of inserting the applicator includes positioning theapplicator in sufficiently close proximity to an endometrium to causeendometrial tissue of the endometrium to absorb the microwave energy;and the step of causing the applicator to emit includes causing theapplicator to emit radiation at a microwave frequency that will causeendometrial tissue to ablate.
 17. The method of claim 16 in which thestep of causing the applicator to emit includes the step of causing theapplicator to emit microwave radiation at 8-12 GHz.